Recently, many efforts are actively made to research and develop OFDM (orthogonal frequency division multiplexing) or OFDMA (orthogonal frequency division multiplexing access) that is suitable for high-speed data transmission over wire/wireless channel. In OFDM, frequency use efficiency is raised using a plurality of carrier waves having mutual orthogonality. A process of modulating/demodulating a plurality of the carrier waves in a transmitting/receiving has the same result of performing IDFT (inverse discrete Fourier transform)/DFT (discrete Fourier transform) and can be implemented at high speed using IFFT (inverse fast Fourier transform)/FFT (fast Fourier transform).
A principle of the OFDM is to reduce relative dispersion in a time domain by multi-path delay spread in a manner of increasing a symbol duration by dividing a high-speed data stream into a plurality of low-speed data streams and by simultaneously transmitting a plurality of the low-speed data streams using a plurality of subcarriers. And, a transmission of data by the OFDM uses a transmission symbol as a unit.
Since the modulation/demodulation in the OFDM can be collectively handled for the entire subcarriers using DFT (discrete Fourier transform), it is unnecessary to design a modulator/demodulator for each of the individual subcarriers.
FIG. 1 is a diagram of a conceptional configuration of an OFDM modulator/demodulator.
Referring to FIG. 1, a serially inputted data stream is transformed into parallel data streams amounting to the number of subcarriers. Inverse discrete Fourier transform (IDFT) is carried out on each of the parallel data streams. For a fast data processing, IFFT (inverse fast Fourier transform) is used. The inverse-Fourier-transformed data is converted to serial data again to be transmitted through frequency conversion. A receiving side receives the corresponding signal to demodulate through a reverse process.
In an OFDM radio communications system according to a related art, a preamble signal inserted in a data frame in a uniform cycle is used as a means for tuning time synchronization and frequency synchronization between a base station and a mobile subscriber station.
FIG. 2 is an exemplary diagram of a data frame in an OFDMA radio communications system according to a related art. In FIG. 2, a horizontal axis is a time axis represented as a symbol unit and a vertical axis is a frequency axis represented as a unit of subchannel. And, the subchannel means a bundle of a plurality of subcarriers.
Referring to FIG. 2, all subcarriers are allocated to a preamble to be modulated and are inserted in the preamble with a uniform interval. Hence, the preamble becomes a reference point of a whole time. Since all signals exchanged between a base station and a mobile subscriber station are inserted by taking the preamble as a reference, acquisition of the preamble is the most basic and important process in exchanging data. A midamble, as shown in FIG. 2, is inserted in the middle of a data frame used in the MIMO (multi-input multi-output) system employing a plurality of antennas for data transmission to play a role similar to that of the preamble.
MIMO is the technology that enables an expectation of remarkable enhancement of frequency efficiency and network link capacity using a plurality of antennas in both transmitting and receiving sides. Recently, attention is paid to the MIMI as an important technology of the mobile communications system demanding for fast data transfer. As a representative MIMO system, there is a BLAST (Bell Laboratory Layered Space-Time) type that can be classified into V-BLAST (vertical-BLAST), H-BLAST (Horizontal-BLAST), D-BLAST (Diagonal-BLAST) and the like.
In the MIMO system, if a transmitting side transmits data to a receiving side via a plurality of antennas, the receiving side receives the transmitted data and estimates channel information per channel. The receiving side finds a weight using the estimated channel information and then feeds back the weight to the transmitting side. The transmitting side applies the fed-back weight to input data to be able to transmit the input data to the receiving side more accurately.
Meanwhile, in the related art OFDM or OFDMA radio communications system, if data to be transmitted to a mobile subscriber station exists, a base station allocates a data region to be transmitted using DL-MAP (download-MAP). The mobile subscriber station receives data via the allocated region (part-‘A’ in FIG. 2). The base station allocates a region (part-‘B’ in FIG. 2) for receiving a feedback of a measured value for a downlink channel status from the mobile subscriber station and designates a channel quality information channel (CQICH). A base station using a multi transmitting antenna designates such information to be fed back as per-channel channel quality, antenna weight, multi-antenna mode selection, permutation selection and the like.
In the related art, in order for the receiving side to estimate the channel information, there exist various reference signals. And, a pilot signal is placed in each predetermined region to enable the receiving side to estimate the channel information. Hence, if the transmitting side transmits data to the receiving side, the receiving side estimates the channel information using the pilot of the data region allocated to itself. The above-explained related art just provides various reference signals (preamble, pilot, data subcarrier, etc.) enabling the estimation of channel information but fails in clearing the information indicating what reference signal or what region will be measured. A channel estimation value may vary according to what reference signal is measured due to various boosting levels, cell loading and frequency reuse factor. In case of the system employing MIMO, as the MIMO midamble is added so that a receiving side enables channel estimation of the midamble as well as a data region allocated to itself, which is not taken into consideration by the related art.
Since the base station preferentially transmits the data to be transmitted to the mobile subscriber station via a non-allocated downlink region without considering a channel status of the mobile subscriber station, data throughput is lowered in case of a poor channel status. Hence, the data region (part-‘A’ in FIG. 2) should be re-selected to transmit via several feedbacks.